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Ceramic-based Multi-Site Platinum Microelectrode Arrays: Morphological Characteristics and Electrochemical Performance for Extracellular Oxygen Measurements in Brain Tissue Ana Ledo, Cátia F. Lourenço, João Laranjinha, Christopher M.A. Brett, Greg A. Gerhardt, and Rui M. Barbosa Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03772 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Analytical Chemistry

Ceramic-based Multi-Site Platinum Microelectrode Arrays: Morphological Characteristics and Electrochemical Performance for Extracellular Oxygen Measurements in Brain Tissue

Ana Ledo1, Cátia F. Lourenço1, João Laranjinha1,2, Christopher M. A. Brett3, Greg A. Gerhardt4 and Rui M. Barbosa§,1,2

1

Center for Neuroscience and Cell Biology, University of Coimbra, Portugal

2

Faculty of Pharmacy, University of Coimbra, Portugal

3

Department of Chemistry, Faculty of Sciences and Technology, University of

Coimbra, Coimbra, Portugal 4

Center for Microelectrode Technology (CenMeT), Department of Neuroscience,

University of Kentucky Medical Center, Lexington, USA

§

Corresponding Author

Rui M. Barbosa Faculty of Pharmacy University of Coimbra Health Sciences Campus Azinhaga de Santa Comba 3000-548 Coimbra, Portugal

[email protected]

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Abstract Ceramic-based multisite Pt microelectrode arrays (MEAs) were characterized for their basic electrochemical characteristics and used for in vivo measurements of oxygen with high resolution in the brain extracellular space. The microelectrode array sites showed a very smooth surface mainly composed of thin-film polycrystalline Pt, with some apparent nano-scale roughness that was not translated into an increased electrochemical active surface area. The electrochemical cyclic voltammetric behavior was characteristic of bulk Pt in both acidic and neutral media. In addition, complex plane impedance spectra showed the required low impedance (0.22 MΩ; 10.8 Ω cm2) at 1 kHz) and very smooth electrode surfaces. The oxygen reduction reaction on the Pt surface proceeds as a single 4-electron reduction pathway at -0.6 V vs Ag/AgCl reference electrode. Cyclic voltammetry and amperometry demonstrate excellent electrocatalytic activity towards oxygen reduction in addition to a high sensitivity (-0.16 ± 0.02 nA µM-1), and a low limit of detection (0.33 ± 0.20 µM). Thus, MEAs provide an excellent microelectrode platform for multi-site oxygen recording in vivo in the extracellular space of the brain, demonstrated in anaesthetized rats, and hold promise for future in vivo studies in animal models of CNS disease and dysfunction.

Keywords: Platinum Microelectrode Array; Oxygen Reduction; Brain Tissue Oxygen.


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Introduction Monitoring brain oxygen levels in vivo has been used over the last decade in patients after brain injury resulting from ischemia, tissue hypoxia, trauma and stroke, among others, allowing the implementation of strategies to maintain adequate levels of tissue oxygen tension (pO2) for improved treatment1-3. More recently, the ability to measure changes in pO2 under conditions of large metabolic and hemodynamic responses such as those observed in epilepsy or traumatic brain injury has become increasingly important in understanding and managing these medical conditions4. Despite the need for in vivo measurement of pO2, the complexity of the cerebral milieu has hindered the achievement of reliable measures. Quantitative measurements of pO2 in vivo have been collected using a number of invasive and noninvasive methods such as i) fiber optic fluorescence ii) near-infrared spectroscopy iii), positron emission tomography (PET), iv) nuclear magnetic resonance (NMR), and v) electron paramagnetic resonance (EPR)2,5-9. The direct measurement of O2 in brain tissue by electrochemical methods with microelectrodes allows for measurements of basal levels and changes of pO2 with high spatial and temporal resolution. In this context, the Clark-type electrode (sensor) technology coupled with amperometry has, for many years, been considered the “gold standard” technique to directly monitor pO2 10. Limitations of this approach include acute tissue damage, O2 consumption by the probe, electrical noise, drift in calibration and slow response time2,11. A wide variety of microelectrodes has been used including noble metal electrodes such as Pt

10,12,13

and Au

14

as well as carbon-based electrodes (e.g. glassy carbon,

carbon paste and carbon fibers)15-17. Platinum has excellent electrode properties for stimulation and recording in neuronal interfacing18 and is recognizably the most active 3 ACS Paragon Plus Environment


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metal towards the electrocatalytic reduction of O2 facilitating the 4-electron reduction to H2 O

19,20

. It is highly biocompatible and inert21-23, allowing for long-term implantation

of Pt-based devices with minimal corrosion-linked allergic reactions as observed with other metals such as Ni or Cu18. Furthermore, its high conductivity is ideal for the design of both stimulation and recording electrodes18,24-26. However, even on Pt-based materials the oxygen reduction reaction (ORR) suffers from sluggish kinetics and requires the use of a high overpotential20,27. Currently, there has been much interest in the development of multiplexed sensors for simultaneous measurements of metabolic markers (e.g. glucose, lactate and O2) from multiple brain areas with high spatial and temporal resolution27. Advances in microfabrication technologies allow for the design of microelectrode array (MEA) platforms comprising multiple Pt sites arranged in a variety of configurations 28. Ceramic-based multi-site Pt MEAs, designed and developed at the Center for Microelectrode Technology (CenMeT), University of Kentucky, USA, are fabricated using photolithographic techniques in well-defined and highly reproducible geometrical configurations. These MEAs have been extensively used for measuring tonic and rapid phasic changes in glutamate levels in anesthetized29 and awake animals30 as well as lactate31 and glucose32. Interestingly, MEAs can also be configured for multi-analyte detection, as described for choline and acetylcholine33. Reports show that these MEAs maintain their recording capabilities during chronic measurements of neurotransmitters and multiple single-unit neuronal activity34-36. The use of a ceramic support material positively enhances the biocompatibility of these implantable MEAs, in addition to providing mechanical strength and electrical inertness37,38. Despite the versatility of the MEA platform, in vivo oxygen measurements in the brain of anesthetized or awake animals using these MEAs with high spatial and 4 ACS Paragon Plus Environment

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temporal resolution have not been investigated to date. In the present work, we extend the previous morphological characterization of MEAs37 and carried out a thorough electrochemical characterization of the latest formulations of these ceramic-based Pt multisite MEAs, which have been mass produced in the thousands, for in vivo recordings of pO2 in the brain extracellular space. We evaluated the general electrochemical properties of the microelectrode arrays using cyclic voltammetry and electrochemical impedance spectroscopy. Finally, we demonstrate the ability of MEAs to record brain pO2 in the extracellular space in anesthetized rats.

Materials and Methods Reagents and Solutions All reagents used were analytical grade and obtained from Sigma-Aldrich. Unless otherwise stated, all in vitro microelectrode evaluations were performed in PBS Lite 0.05 M pH 7.4 with the following composition: 10 mM Na2HPO4, 40 mM NaH2PO4, and 100 mM NaCl. For the electrochemical impedance evaluation, the PBS composition was as follows: 10 mM Na2HPO4, 40 mM NaH2PO4, and 100 mM Na2SO4. Saturated O2 solutions for MEA calibration were prepared by bubbling PBS with 95% O2 at 37 ºC for 20 min, resulting in an O2 solution of 1.0 mM concentration39.

Ceramic-based Platinum Microelectrode Arrays S2 type ceramic-based Pt MEAs were used, supplied by the Center for Microelectrode Technology (CenMet), University of Kentucky, USA), which are commercially available through the Center’s website.

Scanning Electron Microscopy and Elemental Composition Analysis 5 ACS Paragon Plus Environment


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High-resolution scanning electron microscopy (SEM) was performed using a field emission scanning electron microscope coupled with energy dispersive X-ray spectroscopy (EDS) (Zeiss Merlin coupled to a GEMINI II column). The elemental composition was obtained from backscattered electron detection using EDS at 10 keV (Oxford Instruments X-Max). Conductive carbon adhesive tabs were used to ground the MEA surface and secure the sample on the specimen holder.

Electrochemical Instrumentation Electrochemical characterization was performed on a Compactstat Potentiostat (Ivium, The Netherlands) using a three-electrode electrochemical cell comprising the MEA as working electrode, Ag/AgCl in 3M KCl as reference electrode (RE-5B, BAS Inc, IN, USA) and a Pt wire as auxiliary electrode. Amperometric MEA calibrations and recordings in anesthetized rats were performed using a FAST16mkIII potentiostat (Quanteon, KY, USA) in a two-electrode electrochemical cell configuration. For in vivo recordings, the Ag/AgCl in 3M KCl reference electrode was replaced by a miniaturereference electrode produced by electro-oxidation of the exposed tip of a Teflon-coated Ag wire (200 µm o.d., Science Products GmbH, Hofheim, Germany) in 1M HCl saturated with NaCl, which, when in contact with cerebrospinal fluid in the brain containing chloride ions, develops an Ag/AgCl half-cell.

Microelectrode Calibration The S2 MEAs were routinely calibrated to assess performance. Calibrations were performed in 0.05 M PBS Lite pH 7.4 (20 mL) at 37 ºC with continuous stirring at low speed (240 rpm). Oxygen was removed by purging the solution with argon for a minimum period of 30 min, after which the needle was removed from solution and kept 6 ACS Paragon Plus Environment

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above the surface to decrease O2 back-diffusion to the calibration medium. Once a stable baseline was obtained, 4.95 µM aliquots of the O2 saturated solution were added in 5 repetitions (concentration range 0-25 µM). The mean display frequency of the O2 concentration was set at 4 Hz.

Animals All the procedures used in this study were performed in accordance with the European Union Council Directive for the Care and Use of Laboratory animals, 2010/63/EU and were approved by the local ethics committee (ORBEA) and the Portuguese General Direction for Agriculture and Veterinary. One male Wistar rat weighing 300 g (Charles-River Laboratories) was used in these experiments. While in the animal facility, animal husbandry conditions were as follows: housed in pairs in filter-topped type III Makrolon cages in the local vivarium with controlled environmental conditions, including a temperature of 22-24 ºC, relative humidity of 4565%, air exchange rate of 15 times per hour, 12 h light/dark cycle and with standard rat chow diet (4RF21-GLP Mucedola, SRL, Settimo Milanese, Italy) and chlorinated water available ad libitum.

In vivo recording of oxygen in the brain of anesthetized rats. The experimental setup used for amperometric monitoring of O2 in vivo in anesthetized rats was similar to that used in previous studies40. Briefly, the animal was anesthetized with urethane 1.25 g/kg (i.p.) and placed in a stereotaxic apparatus. Body temperature was maintained at 37 ºC with a heated pad coupled to a Gaymar Heating Pump (Braintree Scientific, Inc., USA).



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The skull was exposed by a midline scalp incision and retraction of the skin and temporal muscle. Bleeding was controlled using a Bovie® cautery. A craniotomy was made over the parietal cortex with an area of roughly 7 mm2 (ML: +1 to +4 mm; AP: -2 to -5 mm relative to bregma) with removal of the overlying meninges. The S2 MEA was positioned in the parietal cortex so that the most proximal recording sites (3/4) were localized immediately below the brain surface. An additional small burr-hole was drilled in a site remote from the recording area for the insertion of a miniature Ag/AgCl reference electrode in the subdural space. The cortical surface was maintained wet with saline soaked cotton balls. After insertion of the MEA into the brain, the baseline was allowed to stabilize for at least 60 min. The mean display frequency was set at 100 Hz. Physiological parameters, namely arterial oxygen saturation, heart and breath rates were monitored using the MouseOx Plus pulse oximeter, with a recording frequency of 1 Hz (Starr Life Sciences Corp., PA, USA).

Data Analysis Data analysis was performed using FAST Analysis version 6.0, OriginPro 2016 and GraphPad 5.0. Values are given as the mean ± coefficient of variation (%). The number of repetitions is indicated in each individual determination. Normality of data was confirmed using the D’Agostino & Pearson omnibus normality test (α=0.05). Calculated parameters were statistically evaluated by using an unpaired two-tailed Student’s t-test. Statistical significance was defined as p 99% Pt 37. The Pt surface morphology of S2 MEAs is shown in the SEM micrographs of the planar surface and cross-section of the MEA in Fig. 1B-E. The polyimide layer closely surrounds each Pt pad without covering the recording sites and forms a microwell structure due to the recessed Pt recording pad (Fig. 1B). Despite the apparent surface smoothness (Fig. 1C), the recording sites show some surface irregularities with nanometer size elevations and depression of the surface, reported previously to have an absolute size below 5 nm37. The existence of a nanostructured surface has been demonstrated to improve MEA performance44, possibly due to increased surface area. The cross-section images presented in Fig. 1E revealed a thin layer of the sputtered Pt, with a thickness of around 250 nm, which is in agreement with the specification for manufacturing of the MEAs.

Electrochemically Active Surface Area Considering the nanostructured nature of the surface of the Pt recording site revealed by SEM analysis and the putative contribution of increased electroactive surface area for enhanced electrocatalytic performance, we determined the electrochemical active surface area (ECSA) of the Pt recording sites using two different approaches, as described below. Pt has the ability to undergo hydrogen underpotential deposition (Hupd), a characteristic that allows for the determination of the ECSA by measuring the hydrogen adsorption charge (QH)45. For this purpose, a cyclic voltammogram (CV) was recorded in N2-saturated 0.5 M H2SO4 electrolyte solution and calculated for the negative-going scan after correction for pseudo-capacity in the double-layer region (Fig. 2A). This methodology assumes that the charge under the voltammetric peaks for hydrogen adsorption results from one hydrogen atom per Pt atom of the electrode surface. Using this approach and assuming that the formation of 10 ACS Paragon Plus Environment

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the monolayer of hydrogen at a polycrystalline Pt surface requires 210 µC cm-2

46

, the

mean ECSA of the Pt sites on S2 MEAs was determined to be 5.28 x 10-5 cm2 ± 29.7% (n=13), which corresponds to a surface roughness of 1.06 ± 29.8% (n=13). It is important to mention that staircase cyclic voltammetry is not appropriate for determining the correct charge in time-dependent processes such as proton adsorption47, and thus the current averaging mode of cyclic voltammetry available in the Ivium Compactstat potentiostat was employed. In the second approach, a standard electrochemical redox couple was used to determine the electrochemical behavior of the Pt MEA. Cyclic voltammetry was carried out in 5.0 mM hexammineruthenium(III) chloride (Ru(III)(NH3)6) in 0.5 M KCl solution at scan rates from 25 to 200 mV s-1. As can be observed in Fig. 2B, the CVs revealed a hybrid behavior between conventional cyclic voltammetric and microelectrode behavior48 with well-defined symmetrical oxidation and reduction peaks appearing already at 25 mV s-1. In addition, both the anodic and cathodic peak currents (Ipa and Ipc, respectively) varied linearly with the square root of the scan rate (Fig. 2C; R2 values of 0.996 and 0.998, for Ipa and Ipc, respectively) indicating that the process was diffusion-controlled. The average Ipa/Ipc ratio was 0.89 (n=16), which is close to the theoretical value of 1 for a totally reversible reaction. The mean difference in quartile potentials, |E1/4-E3/4| was 53.8 mV at 25 mV s-1 and did not vary with scan rate, indicating that the reaction is reversible when considering the Tomes criterion of 56.4 mV for a one-electron reversible reaction49. The electrochemically active surface area of the Pt sites of the MEA was estimated using the Randles-Sevick equation for a reversible oxidation-reduction reaction considering a diffusion coefficient of D = 7.1x10-6 cm2 s-1

50

. The calculated



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surface area of the Pt sites was 4.48x10-5 cm2 ± 22.7%, corresponding to a surface roughness of 0.90 ± 22.4% (n=16 sites). As highlighted in Fig. 2C, both methodologies used to determine the ECSA of the Pt sites on the S2 MEA showed that, despite the nanostructured surface suggested by the SEM micrographs, the ECSA is approximately identical to the geometric area. This is a unique property of the Pt-based MEAs and can be attributed to the roughness features being only at the nanoscale level so that the behavior is that of a very smooth electrode. In order to examine the effects of pre-conditioning, different electrochemical cleaning strategies were evaluated. Repeated cycling between -0.6 V and +0.4 V (50 cycles), anodic or cathodic pre-treatment (holding the potential at +1.2 V or -0.6 V vs Ag/AgCl, respectively) produced no significant changes in the CV profiles, only slight changes in Ipa and Ipc. Furthermore, the ESCA determined after anodic pre-treatment showed similar values as with no pre-treatment. This result indicates little to no carbon contamination of the electrode surface. Interestingly, the increased surface roughness of Pt stimulation microelectrodes has been shown to increase Pt electrode performance, namely by decreasing impedance and increasing the current density and charge injection51,52. However, the existence of a smooth surface may be advantageous in impeding the occurrence of undesirable side reactions at the surface such as the reduction or oxidation of the solvent, supporting electrolyte, electrode material, or impurities49.

Electrochemical Behavior in Acidic Electrolyte and in neutral PBS The well-known characteristic cyclic voltammogram of Pt in acid solution was used to further examine the electrochemical behavior of the MEA. The S2 MEA was 12 ACS Paragon Plus Environment

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characterized by cyclic voltammetry in N2-flushed H2SO4 (0.5 M). Fig. 3A shows cyclic voltammograms recorded between -0.25 and 1.2 V vs Ag/AgCl at increasing scan rates (50-1000 mV s-1) of a single Pt site of a S2 MEA. The typical CV exhibited redox peaks at -0.06 and -0.17 V, which could be attributed to strong and weak proton adsorption on Pt surfaces with (100) and (110) basal planes, respectively53,54. Furthermore, the presence of the three distinct peaks for H+ desorption indicate a high quality Pt surface54,55. An oxidation wave is observed for E > 0.5 V due to the formation of Pt-O and Pt-OH oxide species on the Pt surface and there is a strong reduction peak at ca. 0.52 V corresponding to oxide reduction. Although the CV plots obtained in acid electrolyte are of invaluable importance for the evaluation of the Pt surface properties, it is important to characterize the electrode behavior in a neutral physiological-like media such as 0.05 M PBS Lite at pH 7.4, which simulates brain extracellular fluid. As shown in Fig. 3B, the general CV profile was similar in both acid and neutral electrolyte. The expected negative shift in hydrogen adsorption/desorption and Pt-O formation/reduction peaks was accompanied by a decrease in peak current with increasing pH. Furthermore, the amplitude of the potential window remained the same (approx. 1.6 V). An increase in the aqueous solution potential window in buffered neutral electrolyte compared to acidic electrolyte has been described for nanostructured Pt surfaces54, and the fact that we did not observe the same pH dependent effect is probably due to the overall smoothness of the Pt recording sites of the S2 MEAs.

Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) is a powerful tool in the study of the physical and interfacial properties of electrochemical systems, so the impedance 13 ACS Paragon Plus Environment


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characteristics of the Pt MEA were investigated. Spectra were recorded in N2-flushed PBS Lite pH 7.4 at room temperature by applying a sinusoidal wave of amplitude 10 mV between 100 kHz and 0.1 Hz (10 frequencies per decade) at the OCP (+0.332 V vs Ag/AgCl). Prior to recording each spectrum, the electrode was held at this applied potential for 5 minutes. The Bode plot obtained from the data is presented in Fig. 4A. The data were fitted to a Randles circuit56 (see inset of Fig. 4B) consisting of the cell resistance in series with a combination of a constant phase element (CPE) in parallel with the series combination of a charge transfer resistance (Re) and a Warburg impedance element (W). The latter accounts for mass transfer limitations imposed by diffusion, which appear at lower frequencies. The values for the charge transfer resistance, Warburg element and double layer capacitance from fitting to the equivalent electrical circuit are presented in Table 1. The characteristics of the interface between the neural tissue and implanted electrodes are critical. Low impedance and high stability are desirable characteristics of chronically implanted electrodes, since a low impedance guarantees a higher efficiency because less energy is required to pass current to the tissue24,51. Besides use as amperometric sensors, metal electrodes such as this Pt microelectrode array are widely employed in biomedical applications for rapid multiple single-unit recordings and putatively as neural stimulating electrodes24,57. They present advantages over glassencased microelectrodes, such as low impedance at high frequencies. Whilst decreasing the electrode size is highly desirable in order to ensure high spatial resolution of either recording or stimulation, this is accompanied by an increase in the interfacial impedance. The value of Zʹ at 1 kHz is typically reported for impedance measurements on microelectrodes. In this work, the MEAs showed a Zʹ value of 10.8 Ω cm2 (0.217 14 ACS Paragon Plus Environment

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MΩ before area normalization) at 1 kHz. These values are in line with those reported for bulk Pt (5.57 Ω cm2)58 and much higher than those reported for Pt surfaces with increased roughness resulting from deposition of Pt-black (1.12 x 10-9 Ω cm2)59 or conducting polymers such as PEDOT

52

. In an initial paper characterizing this type of

ceramic based Pt MEA, Burmeister et al., reported a Zʹ value of 17.8 ± 2.8 MΩ (445 Ω cm2) at 500 Hz41. At 500 Hz, these newer generation MEAs show a significantly lower impedance value of 0.413 MΩ (20.6 Ω cm2)41, which suggests improved surface characteristics of the new design, better surface cleaning of the Pt during manufacturing and probably less contamination from surface carbon (unpublished data). The CPE exponent is very close to unity, which demonstrates that the Pt surface of the S2 MEA is very smooth, in agreement with the evidence from both the SEM micrographs and the determination of the ECSA.

Oxygen Reduction Reaction at the Pt Surface The oxygen reduction reaction (ORR) has been extensively studied in the context of fuel cell development60,61. In general, the ORR follows one of two reaction pathways: direct 4-electron reduction (reaction 1) or a 2-step 2-electron reduction, with the generation of H2O2 as an intermediate that is further reduced to water (reactions 2 and 3).

+ + → , = .

(1)

+ + → , = .

(2)

+ + → , = .

(3)



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Notwithstanding, little is still understood about electrode kinetics and electrocatalytic properties in physiologically relevant milieu regarding pH, temperature and ionic strength. A better understanding of electrode performance is critical to design improved strategies for long-term stability and performance in vivo. Representative cyclic voltammograms in N2–flushed and air-saturated (0.27 mM O2) PBS Lite recorded at 100 mV s-1 are shown in Fig. 5A. The diffusion-limited current for oxygen reduction is reached at ca. -0.2 V; at -0.45 V proton adsorption begins to predominate. Figure 5B shows a plot of the amperometric current measured at different applied potentials in N2 saturated and air-saturated medium (0.27 mM O2). The subtracted current (inset) indicates an extended plateau region down to -0.6 V vs Ag/AgCl. Since the voltammograms show only one reduction step, it can be inferred that the ORR occurring at the Pt surface of these S2 MEAs appears to be a one-step 4-electron reduction reaction to H2O, as expected from what has been previously described for the Pt surface19. To further establish the most suitable working potential for monitoring O2 in vivo, calibrations were performed at applied potentials ranging from -0.1 to -0.8 V vs Ag/AgCl and both the sensitivity (slope) and LOD were determined. As shown in Fig. 5C, between -0.1 and -0.5 V vs Ag/AgCl, the sensitivity vs potential plot shows an increase in sensitivity reaching a plateau between -0.5 and -0.7 V. The LOD decreases from -0.1 to -0.6 V, and at more negative potentials has a constant value of ca. 0.3 µM. Considering both the plots in Fig. 5 panels A-C and the CV plot in N2-flushed PBS in Fig. 3B, it is clearly observable that -0.7 V is already in the hydrogen evolution region and at the negative limit of the applied potential window in PBS Lite. Thus, the optimal working potential for monitoring O2 was chosen to be -0.6 V vs Ag/AgCl. Fig. 5D 16 ACS Paragon Plus Environment

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shows a representative calibration of an S2 MEA at a holding potential of -0.6 V vs Ag/AgCl as well as the respective calibration curves for each of the 4 sites of the S2 MEA (inset). Also highlighted in the top left corner of Fig. 5D is the rapid response of the 4 channels to the first addition of O2. Thus, despite the slow stirring of the calibration media, one can still observe a rapid response towards oxygen reduction. On average, the Pt MEAs exhibited an oxygen sensitivity of -0.16 nA µM-1 ± 17.7% (R2=0.98 ± 1.87%), a sensitivity/unit area of 3.2 mA mM-1 cm-2 ± 17.7 % and a LOD of 0.33 µM ± 67.6% (n=15). The analytical parameters determined by amperometry at an applied potential of -0.6 V vs. Ag/AgCl are summarized in Table 2, and are compared with those reported in the literature. As shown in Table 2, both the sensitivity and the LOD are in good agreement with those reported both for Pt-based microelectrodes, and also for carbon-based microelectrodes. The sensitivity was found to be one order of magnitude higher than that reported for similar thin-film Pt microelectrode arrays62 or carbon epoxy microelectrodes63 and similar to that reported for carbon paste microelectrodes15 and for Pt/Ir disk microelectrodes64. The LOD is an analytical parameter that is either omitted or not determined in many publications. Although the value reported here (0.33 µM ± 67.5%) is slightly higher than that reported by others64, it must be emphasized that the LOD is highly dependent on the experimental conditions in which the calibrations are performed. Here the oxygen amperometric response of the MEAs was assessed with minimum stirring, since it was previously reported that the oxygen electrode response in brain tissue was independent of flow15. Due to the MEA size and design it was not possible to perform calibrations in a closed vessel. However, increased stirring increased O2 back-diffusion into the PBS Lite following N2-flushing, leading to greater signal noise (thus negatively impacting



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LOD, which is calculated as 3xSD of the baseline) while producing a positive drift in the recorded current. The analytical parameters determined by amperometry at a holding potential of 0.6 V vs Ag/AgCl are summarized in Table 2.

In vivo recording of changes in pO2 in the anesthetized rat brain For in vivo recording in brain tissue, be it in the anesthetized or in the awake freely-moving rodent, high temporal and spatial resolutions are highly desirable with minimal tissue damage. This may be achieved with commercially available modified Clark-type microelectrodes65-67, bare carbon fiber microelectrodes17,68,69 and carbon paste microelectrodes15. To enhance the electrochemical characteristics of carbon fiber microelectrodes, others have carried out surface modifications with carbon nanotubes and/or electrodeposition of Pt70. However, these electrodes allow single-site recordings. The ability to record local pO2 from multiple sites within the brain can be achieved with multisite Pt-MEAs with distinct pad geometries. Ultimately, recording using multisite biomorphic MEA designs (sites placed at specific distances to target specific brain regions) is possible35,71. In order to demonstrate the capability of the S2 MEA to monitor changes in oxygen in vivo in the rat brain, we monitored changes in the local pO2 in the cerebral cortex of an anesthetized rat respiring air, O2 and argon. As shown in Fig. 6, changing the pO2 in the respired air produced changes in arterial blood O2 saturation (bottom trace, in blue) that were accompanied by similar changes in local pO2. For simplicity, only 2 channels are shown, one from each side-by-side pair of the MEA. The higher basal pO2 recorded from the site closest to the surface is most likely a result of O2 diffusion from the surrounding atmosphere in the exposed tissue. 18 ACS Paragon Plus Environment

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Conclusions

Ceramic-based multisite Pt MEAs were used for measurements of oxygen in vitro and in vivo in the brain extracellular space with high temporal and spatial resolution. The microelectrode array sites showed a very smooth surface mainly composed of a thin-film polycrystalline Pt (95%). The apparent nano-scale roughness of the Pt surface does not lead to an increased electrochemically active surface area, but the active surface area is proportional to the geometric area of the Pt surface. This is a unique feature of the MEAs that is often not seen for other microelectrodes. Investigation of the electrochemical behavior of the Pt MEAs by cyclic voltammetry shows the characteristics of bulk Pt in acidic and neutral media. In addition, complex plane impedance spectra showed the necessary low impedance values ((0.22 MΩ; 10.8 Ω cm2) at 1 kHz) and that the electrode surfaces are very smooth. The oxygen reduction reaction on the Pt surface proceeds as a 4-electron reduction pathway at -0.6 V vs Ag/AgCl. Electrochemical evaluation by cyclic voltammetry and amperometry evidences an excellent electrocatalytic activity towards oxygen reduction in addition to a high sensitivity (-0.16 ± 0.02 nA µM-1), and a low limit of detection (0.33 ± 0.20 µM). Thus, MEAs provide an excellent microelectrode platform for multisite oxygen recording in vivo in the extracellular space of the brain.



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Acknowledgements This work is funded by FEDER funds through the Operational Program Competitiveness Factors - COMPETE and national funds by FCT - Foundation for Science and Technology under the strategic project UID/NEU/04539/2013. C.F.L. acknowledges fellowship SFRH/BPD/82436/2011 from FCT. We acknowledge Quanteon, LLC for salary support of A.L.

Conflict of interest G.A.G. is the sole proprietor of Quanteon, LLC, which makes the Fast16 recording system used for control of the MEA technology.


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Tables Table 1 – Summary of fitted parameter results for impedance spectroscopy measurements for the Pt MEAs. Values shown with * are normalized by surface area. E (V)

Rs (kΩ)

+0.332 (OCP)

3.5

Re (MΩ) *Re (kΩ cm2) 16.0 *0.8

CPE, nF sn-1 *CPE (µF cm-2 sn-1)

n

W (x106) (1/Ω s-0,5)

1.2 *24.02

0.99897

0.7

Table 2 – Analytical performance comparison of different microelectrodes towards the oxygen response in vitro. Data are presented as mean ± SD.

Microelectrode Type

Sensitivity (nA µM-1)

Linearity (R2)

Sensitivity/unit area (mA.mM-1.cm-2)

LOD (µM)

References

Thin film Pt MEA (n=12)

-0.16 ± 0.02

0.98±0.02

-3.2 ± 0.5

0.33 ± 0.2

Current study

Thin film Pt MEA b (n=5)

-0.58 +/- 0.001a

nd

nd

[59]

Pt/Ir disk (n=4-18)

-1.43 +/- 0.05

0.979

-9.1 +/- 0.6

0.08 +/0.01

[61]

Carbon paste disk (CPE) (n=4-16)

-1.09 ± 0.03

0.998±0.01

-4.8 +/- 0.2

0.09 +/0.01

[61]

Pt/VACNT-CFc

-0.91

0.995

nd

nd

[62]

Pt/FCNA modified GCE d

nd

nd

-4.1

nd

[63]

Carbon Epoxy (n=6)

-0.222 ± 0.017

0.992

-0.148 +/ 0.011 a

Ceramic-Based Multisite Platinum Microelectrode ... - ACS Publications

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